Most materials expand when the temperature is increased. However, there are some materials that contract or expand only very little with increasing temperature. One of the best known examples of a product composed of glass-ceramic without significant thermal expansion is Ceran® from Schott and is used, for example, to produce a glass-ceramic cooking area. Such a cooking area contains the main constituents SiO2, Al2O3 and Li2O. The expansion behavior is decisively achieved by the crystallization of lithium aluminosilicates having low or negative expansion. In the relevant temperature range, such materials have a coefficient of thermal expansion close to zero. Even numerous thermal stressing cycles and temperature shocks bring about only very low thermal stresses in such materials, which usually do not lead to mechanical failure, i.e., fracture. A disadvantage is that very high process temperatures, sometimes above 1600° C., have to be generated in the production of such glass-ceramics.
Such zero-expansion materials are also used for precision applications, e.g., large-area telescope mirrors or in microoptics. Those materials are naturally conceivable in many fields of application in which thermal shock resistance over a more or less wide temperature range is required, e.g., in the case of cookware.
The abovementioned negative or very low thermal expansion can be achieved by crystalline phases having low or negative expansion, e.g., β-spodumene (high-temperature modification of LiAlSi2O6), β-eucryptite (LiAlSiO4), cordierite (Mg2Al2Si5O18), high-temperature quartz or β-quartz mixed crystals. These are well known and described in detail in various articles (e.g., in J. N. Grima, V. Zammit, R. Gatt, Xjenza 11, 2006, 17-29 or in G. D. Barrera, J. A. O. Bruno, T. H. K. Barron, N. L. Allan, J. Phys.: Condens. Matter 17, 2005, R217-R252).
Apart from various glass-ceramics, fused silica also has a very low coefficient of expansion (0.5·10−6 K−1) that can be reduced further by doping with TiO2. However, production of that material is difficult, complicated and therefore expensive because of the extremely high melting temperatures (>2200° C.).
Further crystalline phases having a negative thermal expansion are ZrW2O8, HfW2O8, ZrV2O7 and HfV2O7 as described in WO 02/22521 A1. However, nothing has been said hitherto about glasses from which those phases were crystallized and then have a coefficient of thermal expansion close to zero. In addition, those materials are usually expensive (especially in the case of tungsten compounds) and can contain toxic components (vanadium compounds). Further publications describing materials displaying negative or low expansion are indicated below.
U.S. Pat. No. 5,919,720 A describes a variety of phases of the formula A2-x3+Ay4+Mz3+M3-y6+PyO12 having negative and low coefficients of expansion. In US '720, ScHoW3O12 and Al1.5In0.5W3O12, for example, have coefficients of expansion of −7·10−6 K−1 and 1·10−6 K−1. Those components are phases which can be produced, for example, via solid-state reactions. However, production of conventional silicate or borosilicate glasses from which large amounts of those phases crystallize does not appear to be possible. It can therefore be assumed that those phases cannot be crystallized from glasses in volume concentrations that would be high enough to bring about a very low coefficient of thermal expansion in the overall material.
Glass-ceramics having a low expansion, e.g., such as Ceran® are generally produced by melting and subsequent crystallization of glasses from the system Li2O—Al2O3—SiO2. Such lithium aluminosilicate glasses are, for example, described in EP 0 995 723 B1, US RE 29 437 E and DE 39 27 174 A1, but they all require particularly high melting temperatures.
In some publications, alkaline earth metal oxides are possible as additives. However, those oxides are not added to influence the crystal phase, but merely to alter the properties of the glass.
DD 142 037 A1 describes a lithium aluminosilicate glass in which BaO is used to improve the crystallization behavior and reduce the viscosity. However, BaO is known to bring about a significant increase in the coefficient of expansion of a glass.
DE 21 32 788 C also describes glasses doped by incorporation of up to 2% by weight of BaO and/or CaO and up to 12% by weight of rare earth metal oxides, but those increase the coefficient of expansion. Here too, the negative expansion is achieved, inter alia, by crystals having the same structure type as β-quartz. The high-temperature quartz structure is stabilized by aluminum oxide and also monovalent (Li2O, Na2O) or divalent (ZnO, MgO) oxides. Very high melting temperatures of 1550 to 1600° C. are mentioned.
WO 2005/009 916 A1 states that the negative expansion can be achieved by crystallization of β-eucryptite and β-quartz. Both ZnO and also BaO and/or SrO may be added. However, incorporation of those constituents into crystal phases is not mentioned. In the working examples, a minimum melting temperature of 1480° C. is indicated.
In general, it can be concluded that alkaline earth metal oxides such as BaO or SrO have hitherto been added to the glasses only to reduce the melting temperature, perhaps also to suppress the tendency for crystallization to occur. This addition could be made only in small amounts since oxides of this type increase the coefficient of thermal expansion of the glass-ceramics.
It can be seen from the above overview in the field of materials having low or negative expansion that only a very limited number of crystalline phases having no thermal expansion or a negative thermal expansion in the temperature range T>20° C. are known. Mainly phases that can be crystallized from aluminosilicate glasses are used commercially. They are usually based on the Li2O—Al2O3—SiO2 system. However, glasses of that system have very high melting temperatures, usually significantly above 1550° C. This accordingly leads to high energy costs and in particular to a high degree of technical complication.
All published silicate compositions having a low or negative coefficient of thermal expansion are aluminosilicate compositions having the disadvantages indicated above.
It could therefore be helpful to provide a possible way of producing ceramics and/or glass-ceramics having a low or else negative thermal expansion with a very small outlay at a low melting temperature.